(1) Field
The disclosed methods and systems relate generally to electrostatic spray devices, and more particularly to an electrospray ion source assembly using a porous electrochemical flow-cell.
(2) Description of Relevant Art
The electrospray (ES) process generally includes flowing a sample liquid into an electrospray ion source comprising a small tube or capillary which is maintained at a high voltage, in absolute value terms, with respect to a nearby surface. The liquid introduced into the tube or capillary is dispersed and emitted as fine electrically charged droplets (plume) by an electrical field between the tube and the surface. The ionization mechanism generally involves the desorption at atmospheric pressure of ions from the fine electrically charged particles. The ions created by the electrospray process can then be used for a variety of applications, such as mass analysis in a mass spectrometer.
The electrospray ion source operates electrolytically in a fashion analogous to a two-electrode controlled current (CCE) flow cell, effectively forming an electrochemical cell in a series circuit A metal capillary or other conductive contact (usually stainless steel) placed at or near the point from which the charged ES droplet plume is generated (the ES emitter) acts as the working electrode in the system.
One issue with conventional electrospray systems is that the electrochemical reactions (e.g., electrolysis reactions) that take place in the electrospray (ES) emitter can influence the gas-phase ions formed and ultimately analyzed by the mass spectrometer, because they may change the composition of the solution from that which initially enters the ion source to include unwanted ionized compounds (e.g., analytes). These changes include, but are not limited to, analyte electrolysis resulting in ionization of neutral analytes or modification in the mass or charge of the original analyte present in solution, changes in solution pH through electrolytic H+or OH−production/elimination, and/or the introduction/elimination of specific species to/from solution (e.g., introduction of Fe2+, ions from corrosion of a stainless steel emitter).
Another issue with conventional electrospray systems is that the compounds (e.g., analytes) most amenable to ionization through the electrospray process (and thus most amenable to analysis through the ES process) are ionic compounds, and/or compounds that can be ionized through acid/base reactions. Other compounds, such as neutral and non-polar compounds may not as readily be ionized by the ES process, and thus may not be easily processed and analyzed using apparatus and techniques that require that the compounds to be analyzed be ionized and electrosprayed.
Yet another concern with conventional ES systems relates to controlling the potential of the emitter electrode. As the system is a CCE process, the emitter potential is dictated by solution composition, flow rate, and the applied voltage. As these factors change so too does the emitter potential, making it difficult to control various electrochemical reactions in the system. Another factor that can affect the ES process is low analyte electrolytic efficiency resulting from the use of traditional emitter electrode designs. In many instances a low emitter analyte electrolytic efficiency will result in sub optimal signal intensities and/or a distribution of products.